Rain and wind from Typhoon No. 5 (Noru) captured by GCOM-W

According to the announcement by the Japan Meteorological Agency (JMA), Typhoon No. 5 “Noru”, reached typhoon strength on July 21, 2017, and is expected to approach southern Kyushu Island and the Amami region while maintaining its strength until the morning of August 5. At 14:00 (JST) on August 4, Typhoon No. 5 was almost stationary over the sea, around 190 km east of Amami Oshima, with a central pressure of 950 hPa and maximum winds of 40.0 m/s near the center of typhoon. Figure 1 shows the cloud distribution of Typhoon No. 5 at 14:00 (JST) on August 4, as captured by the JMA’s geostationary weather satellite “Himawari-8” and visualized using the JAXA Himawari Monitor.

While Himawari-8 can continuously observe the cloud distribution caused by typhoons in detail, it cannot measure the rainfall or wind speeds near the sea surface through the typhoon clouds.

Figure 2 shows the changes in rainfall amount associated with Typhoon No. 5 from July 30 to August 4, as observed by the Advanced Microwave Scanning Radiometer 2 (AMSR2) on board the JAXA’s Global Change Observation Mission – Water (GCOM-W, or “SHIZUKU”). This data has been overlaid on the cloud distribution observed by Himawari-8 at the same time. A microwave radiometer is an instrument that detects weak microwaves (radio waves) emitted from or scattered by raindrops and ice particles in the atmosphere, as well as emitted from the Earth’s  surfaces. It can capture features such as the structure of rain beneath clouds and strength of winds, which cannot be measured by Himawari-8. AMSR2 observations show that, over time, the rainy areas in the central cloud region caused by the typhoon organized into a donut shape around the eye of the typhoon. By August 4, the rainy areas had become concentrated on the eastern side of the eye.

Figure 3 shows the changes in all-weather sea surface wind speed observed by AMSR2 during the same period as Figure 2. All-weather sea surface wind speed product can retrieve wind speeds even beneath typhoons by using frequency bands that are less affected by clouds. Around July 30, the wind speed just under the typhoon was almost 60 m/s, but, as time passed, its distribution and intensity changed. By August 4, the wind speeds near the eye of the typhoon became to 30–40 m/s, so caution was required for both rain and wind.

The GCOM-W satellite can observe the area around Japan twice a day with a swath width of about 1,600 km, unaffected by clouds and regardless of day or night, making it well suited for monitoring changes in the distribution of rain and wind caused by typhoons. In addition, GCOM-W observation data are used as input data for numerical weather prediction by JMA and various meteorological agencies around the world, and are utilized to improve the accuracy of weather forecasts.